It's not just you, hasn't worked for me since June of 2017, and I stopped using Adium because of this.

I have a strong urge to talk to some people on Jabber now (Pi day 2018), just installed Adium and tried it again, no joy. There is more information out there about ATS - Application Transport Security - than when I first started getting errors. I can't figure out how to make the exceptions stuff work with Adium, as the Info.plist is not a plain text file one can edit. And I'm not sure that is such a good idea for many reasons.

It's really disheartening, Adium was the best Jabber client, but only when the hideous defaults could be tamed. I'm on the autism spectrum and programs that do cutesy stuff to get my attention DO - in the form of me immediately uninstalling them if there isn't a way to turn off nuisance behavior.